In a typical data communication system data is sent from a transmitter to a receiver over a communication media such as a wire, a fiber optic cable or air. In some applications, data is transmitted over the media via a modulated radio frequency (“RF”) signal. For example, in a wireless communication system a transmitter may use a relatively low frequency (e.g., less than 1 MHz) data signal to modulate a relatively high frequency carrier signal (e.g., 1 GHz). The receiver will then demodulate the received modulated carrier signal to extract the original data signal.
Typical forms of modulation used in wireless communication include phase modulation and amplitude modulation. Here, phase modulation may involve adjusting the phase of the carrier signal according to the information in the data signal. In contrast, amplitude modulation may involve adjusting the amplitude of the carrier signal according to the information in the data signal. Conventionally, the amplitude waveform of a modulated signal is referred to as the “envelope” of the signal.
In some applications it may be desirable to use a form of modulation that does not modulate the envelope of the carrier signal. For example, when constant envelope modulation is employed in an RF system, the system may use a nonlinear power amplifier instead of a linear power amplifier. This may provide, as a result, a more cost effective and/or efficient system.
An example of a constant envelope system is defined by the Global System for Mobile communications/General Packet Radio Service (“GSM/GPRS”) standard for wireless communication devices. This standard incorporates Gaussian Minimum Shift Keying (“GMSK”) modulation.
In general, a constant envelope modulation signal may be represented as:s(t)=A cos [ωct+Φ(t)]  EQUATION 1
where Φ(t) contains the information of the signal.
Since the envelope of the signal is constant, the transmitter architecture is not limited to a Cartesian topology. As a result, other architectures have been proposed for constant envelope systems.
For example, FIG. 1 illustrates a transmitter 100 implemented using a translational loop (also know as an offset PLL) architecture. A modulator 102 modulates an input signal (e.g., I/Q signals 104) and outputs a modulated signal 106 to a limiter 108. The limiter 108 serves to remove any amplitude information present in the signal 106. Thus, the signal 110 output by the limiter 108 may, for example, only include phase information.
A phase lock loop circuit modulates an output signal 112 according to the signal 110. The phase lock loop circuit includes a phase/frequency detector and charge pump 114, a low pass filter 116, a voltage controlled oscillator (“VCO”) 118 and a feedback loop. The feedback loop includes a mixer 120 and a low pass filter 124 for downconverting the output signal 112 using to a local oscillator (“LO”) signal 122.
Although a constant envelope architecture may enable the use of more efficient components such as nonlinear power amplifiers and may be used effectively in circuits such as a translational loop, this architecture may not efficiently use the available bandwidth of the communication media. To facilitate efficient transmission of data over the media, more than one form of modulation may be used to modulate a signal. For example the EDGE standard incorporates both phase and amplitude modulation. As a result, the EDGE standard may support data rates three times higher than GSM/GPRS while using the same bandwidth. In this case, the modulated signal may be represented as:s(t)=A(t)cos [ωct+Φ(t)]  EQUATION 2
To obtain the benefits of using a nonlinear power amplifier and a translational loop, the architecture of FIG. 1 may be modified to a polar transmitter architecture as shown in FIG. 2. In this case, a modulator 202 provides an input signal 204 that may be phase and amplitude modulated. The phase and envelope information may then be separated in the baseband.
For example, a limiter 206 may provide the phase information (cos [ωct+Φ(t)]) to a constant amplitude transmitter such as the translational loop portion of the transmitter 200 of FIG. 2. As in FIG. 1, the transmitter 200 may include translational loop components such as a phase/frequency detector and charge pump 208, a low pass filter 210, a VCO 212, a mixer 214 and a low pass filter 216 that outputs a phase modulated signal 222.
An envelope detector 220 may detect the envelope (e.g., amplitude modulation) of the modulated input signal 204. The envelope of the signal (A(t)) is used to amplitude modulate the signal 222 by controlling the gain of the power amplifier 218. Thus, the power amplifier 218 outputs a phase and amplitude modulated signal 224.
Although this architecture may provide benefits as discussed above, the performance of a transmitter implementing this architecture may not be optimum due to limitations in the system. Accordingly, a need exists for an improved transmitter for transmitting modulated signals.